I was warned that some plotholders have lost hundreds of leeks to the onslaught from leek moths. They lay their eggs on the outer green leaves and the larvae burrow under the surface and make their way down to the roots. They colonise the inside of the leek and you end up with nothing.
So I inspected my leeks and found the damage on one or two outer leaves. I cut them off and burned them.
The RHS website says that there is no known prevention measures other than very fine netting, and that an insecticide might do some good, but nicotine is best. But....nicotine is only available to commercial growers. I just hope I caught mine in time.